It is common practice in the production of carbon and graphite electric furnace electrodes to employ a calcined petroleum coke (i.e., raw petroleum coke that has been heated to temperatures above about 1200.degree. C.) as the filler or aggregate material and to mix this filler or aggregate with a carbonaceous binder such as pitch. The mixture is formed into the shape of the electrode, either by molding or extrusion, and is then baked at an elevated temperature sufficient to carbonize the binder (e.g. about 800.degree. C.). In those cases where a graphitized electrode is required, the baked electrode is further heated to temperatures of at least about 2800.degree. C.
Petroleum coke particles have a tendency to "puff", that is, to expand and even to split when heated to temperatures above about 1500.degree. C., if they contain more than about 0.3% by weight sulfur. Electrodes made from such cokes lose density and strength and sometimes split lengthwise when heated to these high temperatures. As indicated, graphite electrodes are normally heated to at least 2800.degree. C. during their manufacturing process. Carbon electrodes, which are not graphitized during the manufacturing process, reach temperatures between about 2000.degree. C. and 2500.degree. C. during their use in silicon or phosphorus furnaces.
Puffing is associated with the release of sulfur from its bond with carbon inside the coke particles. If the sulfur containing vapors cannot escape from the particles or from the electrode fast enough, they create internal pressure which, in turn, increases the volume of the particles and may cause the electrode to split.
The conventional remedy for puffing has been to add an inhibitor such as iron oxide or other metal compound to the coke-pitch mixture before the electrodes have been formed. It has been shown, for example, that about 2 weight percent iron oxide can be effective to reduce coke puffing. Some cokes that have a higher tendency to puff or start puffing at a lower temperature cannot be adequately controlled by iron oxide.
Various attempts have been made to provide other improved puffing inhibition methods which overcomes the above and other disadvantages of the prior art. For example, in U.S. Pat. No. 2,814,076 issued to J. W. Gartland on Nov. 26, 1957, there is disclosed an improved method of producing graphite articles such as electric furnace electrodes wherein an alkali metal compound from group I of the Periodic table, notably sodium carbonate, is employed as a puffing inhibitor. The sodium carbonate may be added to the article by impregnating the article after baking with a solution of the sodium carbonate or by adding the puffing inhibitor directly to the coke-pitch mix. Although adding sodium carbonate to the coke-pitch mix is more convenient than adding it to the baked article, this method produces a finished electrode of inferior quality, i.e., lower density and lower strength.
Another problem encountered when the puffing inhibitor is added directly to the coke-pitch mix is that sodium carbonate reacts with acidic extrusion aids which may be employed in the mix. Unfortunately, this reaction often causes extrusion problems leading to poor structure of the electrode.
Another approach to solving the problem of coke puffing in the production of carbon and graphite electrodes is disclosed in U.S. Pat. No. 3,506,745 issued to L. H. Juel et al. on Apr. 14, 1970. In this approach, high sulfur petroleum coke particles are treated prior to their incorporation in a carbonaceous mix by contacting the coke particles with a puffing inhibitor and heating the particles in a substantially non-oxidizing atmosphere to temperatures above about 1400.degree. C., and also above that at which the coke begins to puff in the absence of the puffing inhibitor and preferably above 2000.degree. C. The puffing inhibitor may be introduced by dusting fine powders of the inhibitor onto the granular petroleum coke or an aqueous slurry containing the inhibitor may be prepared and sprayed onto the coke before heating the coke particles to puffing temperatures. The coke particles are then cooled to about ambient temperatures and blended with a pitch binder to form a conventional carbonaceous mix. The puffing inhibitor combines with the sulfur and is volatilized when the coke is heated to puffing temperatures and above. The problem with this approach is that the process requires heating the coke particles to temperatures that are significantly higher than those ordinarily employed during the usual calcining process. Consequently, this treatment can only be carried out with a process which is different from ordinary calcining practices, consuming more energy and requiring more expensive equipment.